【蓝牙sbc协议】sbc源码阅读笔记（一）—

sbcenc.c

main(int argc, char *argv[])

首先设定 option 的默认值，然后根据用户命令设定 option 相关参数。

对指定文件进行编码（执行encode函数）

usage(void)

打印相关option:

Option	Option	打印帮助信息
h	help	打印帮助信息
v	verbose	详细模式
m	msbc	mSBC编解码器
s	subbands	子带数量（4/8）
b	bitpool	Bitpool value
j	joint	联合立体声
d	dualchannel	双声道
S	snr	信噪比模式（default/loudness）
B	blocks	block数量（4/8/12/16）

encode(char *filename, int subbands, int bitpool, int joint, int dualchannel, int snr, int blocks, bool msbc)

定义一个 au_header类型的变量au_hdr

// formats.h
struct au_header {uint32_t magic;       /* '.snd' */uint32_t hdr_size;    /* size of header (min 24) */uint32_t data_size;    /* size of data */uint32_t encoding;    /* see to AU_FMT_XXXX */uint32_t sample_rate;   /* sample rate */uint32_t channels; /* number of channels (voices) */
};

定义一个sbc_t类型的变量sbc

// sbc.h
struct sbc_struct {unsigned long flags;uint8_t frequency;uint8_t blocks;uint8_t subbands;uint8_t mode;uint8_t allocation;uint8_t bitpool;uint8_t endian;void *priv;void *priv_alloc_base;
};
typedef struct sbc_struct sbc_t;

定义ssize_t类型的变量encoded和len，实际是ssize_t就是long类型。

// _types.h
typedef long __darwin_ssize_t; /* byte count or error */
// _ssize_t.h
typedef __darwin_ssize_t ssize_t;

检测au_hdr的长度是否是24。

if (sizeof(au_hdr) != 24) {/* Sanity check just in case */fprintf(stderr, "FIXME: sizeof(au_hdr) != 24\n");return;
}

确定指定文件合法性并打开文件。

if (strcmp(filename, "-")) {fd = open(filename, O_RDONLY);if (fd < 0) {fprintf(stderr, "Can't open file %s: %s\n",filename, strerror(errno));return;}
} elsefd = fileno(stdin);

将文件中头部数据读入au_hdr，并确定是否读取成功。

len = read(fd, &au_hdr, sizeof(au_hdr));
if (len < (ssize_t) sizeof(au_hdr)) {if (fd > fileno(stderr))fprintf(stderr, "Can't read header from file %s: %s\n",filename, strerror(errno));elseperror("Can't read audio header");goto done;
}

确定文件的格式是否符合Sun/NeXT audio S16_BE格式。

if (au_hdr.magic != AU_MAGIC ||BE_INT(au_hdr.hdr_size) > 128 ||BE_INT(au_hdr.hdr_size) < sizeof(au_hdr) ||BE_INT(au_hdr.encoding) != AU_FMT_LIN16) {fprintf(stderr, "Not in Sun/NeXT audio S16_BE format\n");goto done;
}

判断使用sbc编码还是msbc编码。

sbc

初始化sbc。

// sbc.c
// sbc_init()
SBC_EXPORT int sbc_init(sbc_t *sbc, unsigned long flags)
{if (!sbc)return -EIO;memset(sbc, 0, sizeof(sbc_t));sbc->priv_alloc_base = malloc(sizeof(struct sbc_priv) + SBC_ALIGN_MASK);if (!sbc->priv_alloc_base)return -ENOMEM;sbc->priv = (void *) (((uintptr_t) sbc->priv_alloc_base +SBC_ALIGN_MASK) & ~((uintptr_t) SBC_ALIGN_MASK));memset(sbc->priv, 0, sizeof(struct sbc_priv));sbc_set_defaults(sbc, flags);return 0;
}// sbc_set_defaults()
static void sbc_set_defaults(sbc_t *sbc, unsigned long flags)
{struct sbc_priv *priv = sbc->priv;if (priv->msbc) {priv->pack_frame = msbc_pack_frame;priv->unpack_frame = msbc_unpack_frame;} else {priv->pack_frame = sbc_pack_frame;priv->unpack_frame = sbc_unpack_frame;}sbc->flags = flags;sbc->frequency = SBC_FREQ_44100;sbc->mode = SBC_MODE_STEREO;sbc->subbands = SBC_SB_8;sbc->blocks = SBC_BLK_16;sbc->bitpool = 32;
#if __BYTE_ORDER == __LITTLE_ENDIANsbc->endian = SBC_LE;
#elif __BYTE_ORDER == __BIG_ENDIANsbc->endian = SBC_BE;
#else
#error "Unknown byte order"
#endif
}

设置采样率、子带数量等参数。

switch (BE_INT(au_hdr.sample_rate)) {case 16000:sbc.frequency = SBC_FREQ_16000;break;case 32000:sbc.frequency = SBC_FREQ_32000;break;case 44100:sbc.frequency = SBC_FREQ_44100;break;case 48000:sbc.frequency = SBC_FREQ_48000;break;}srate = BE_INT(au_hdr.sample_rate);sbc.subbands = subbands == 4 ? SBC_SB_4 : SBC_SB_8;if (BE_INT(au_hdr.channels) == 1) {sbc.mode = SBC_MODE_MONO;if (joint || dualchannel) {fprintf(stderr, "Audio is mono but joint or ""dualchannel mode has been specified\n");goto done;}} else if (joint && !dualchannel)sbc.mode = SBC_MODE_JOINT_STEREO;else if (!joint && dualchannel)sbc.mode = SBC_MODE_DUAL_CHANNEL;else if (!joint && !dualchannel)sbc.mode = SBC_MODE_STEREO;else {fprintf(stderr, "Both joint and dualchannel ""mode have been specified\n");goto done;}sbc.endian = SBC_BE;sbc.bitpool = bitpool;sbc.allocation = snr ? SBC_AM_SNR : SBC_AM_LOUDNESS;sbc.blocks = blocks == 4 ? SBC_BLK_4 :blocks == 8 ? SBC_BLK_8 :blocks == 12 ? SBC_BLK_12 : SBC_BLK_16;

mSBC

初始化sbc。

// sbc.c
SBC_EXPORT int sbc_init_msbc(sbc_t *sbc, unsigned long flags)
{struct sbc_priv *priv;if (!sbc)return -EIO;memset(sbc, 0, sizeof(sbc_t));sbc->priv_alloc_base = malloc(sizeof(struct sbc_priv) + SBC_ALIGN_MASK);if (!sbc->priv_alloc_base)return -ENOMEM;sbc->priv = (void *) (((uintptr_t) sbc->priv_alloc_base +SBC_ALIGN_MASK) & ~((uintptr_t) SBC_ALIGN_MASK));memset(sbc->priv, 0, sizeof(struct sbc_priv));priv = sbc->priv;priv->msbc = true;sbc_set_defaults(sbc, flags);sbc->frequency = SBC_FREQ_16000;sbc->blocks = MSBC_BLOCKS;sbc->subbands = SBC_SB_8;sbc->mode = SBC_MODE_MONO;sbc->allocation = SBC_AM_LOUDNESS;sbc->bitpool = 26;return 0;
}

读入文件数据，并显示详细信息

/* Skip extra bytes of the header if any */
if (read(fd, input, BE_INT(au_hdr.hdr_size) - len) < 0)goto done;if (verbose) {fprintf(stderr, "encoding %s with rate %d, %d blocks, ""%d subbands, %d bits, allocation method %s, ""and mode %s\n",filename, srate, blocks, subbands, bitpool,sbc.allocation == SBC_AM_SNR ? "SNR" : "LOUDNESS",sbc.mode == SBC_MODE_MONO ? "MONO" :sbc.mode == SBC_MODE_STEREO ?"STEREO" : "JOINTSTEREO");
}

获取编码长度codesize及帧数nframes。

codesize = sbc_get_codesize(&sbc);
nframes = sizeof(input) / codesize;

// sbc.c
SBC_EXPORT size_t sbc_get_codesize(sbc_t *sbc)
{uint16_t subbands, channels, blocks;struct sbc_priv *priv;priv = sbc->priv;if (!priv->init) {subbands = sbc->subbands ? 8 : 4;if (priv->msbc)blocks = MSBC_BLOCKS;elseblocks = 4 + (sbc->blocks * 4);channels = sbc->mode == SBC_MODE_MONO ? 1 : 2;} else {subbands = priv->frame.subbands;blocks = priv->frame.blocks;channels = priv->frame.channels;}return subbands * blocks * channels * 2;
}

编码。

while (1) {unsigned char *inp, *outp;/* read data for up to 'nframes' frames of input data */size = read(fd, input, codesize * nframes);if (size < 0) {/* Something really bad happened */perror("Can't read audio data");break;}if (size < codesize) {/* Not enough data for encoding even a single frame */break;}/* encode all the data from the input buffer in a loop */inp = input;outp = output;while (size >= codesize) {len = sbc_encode(&sbc, inp, codesize,outp, sizeof(output) - (outp - output),&encoded);if (len != codesize || encoded <= 0) {fprintf(stderr,"sbc_encode fail, len=%zd, encoded=%lu\n",len, (unsigned long) encoded);break;}size -= len;inp += len;outp += encoded;}len = write(fileno(stdout), output, outp - output);if (len != outp - output) {perror("Can't write SBC output");break;}if (size != 0) {/** sbc_encode failure has been detected earlier or end* of file reached (have trailing partial data which is* insufficient to encode SBC frame)*/break;}
}

// sbc.c
SBC_EXPORT ssize_t sbc_encode(sbc_t *sbc, const void *input, size_t input_len,void *output, size_t output_len, ssize_t *written)
{struct sbc_priv *priv;int samples;ssize_t framelen;int (*sbc_enc_process_input)(int position,const uint8_t *pcm, int16_t X[2][SBC_X_BUFFER_SIZE],int nsamples, int nchannels);if (!sbc || !input)return -EIO;priv = sbc->priv;if (written)*written = 0;if (!priv->init) {priv->frame.frequency = sbc->frequency;priv->frame.mode = sbc->mode;priv->frame.channels = sbc->mode == SBC_MODE_MONO ? 1 : 2;priv->frame.allocation = sbc->allocation;priv->frame.subband_mode = sbc->subbands;priv->frame.subbands = sbc->subbands ? 8 : 4;priv->frame.block_mode = sbc->blocks;if (priv->msbc)priv->frame.blocks = MSBC_BLOCKS;elsepriv->frame.blocks = 4 + (sbc->blocks * 4);priv->frame.bitpool = sbc->bitpool;priv->frame.codesize = sbc_get_codesize(sbc);priv->frame.length = sbc_get_frame_length(sbc);sbc_encoder_init(priv->msbc, &priv->enc_state, &priv->frame);priv->init = true;} else if (priv->frame.bitpool != sbc->bitpool) {priv->frame.length = sbc_get_frame_length(sbc);priv->frame.bitpool = sbc->bitpool;}/* input must be large enough to encode a complete frame */if (input_len < priv->frame.codesize)return 0;/* output must be large enough to receive the encoded frame */if (!output || output_len < priv->frame.length)return -ENOSPC;/* Select the needed input data processing function and call it */if (priv->frame.subbands == 8) {if (sbc->endian == SBC_BE)sbc_enc_process_input =priv->enc_state.sbc_enc_process_input_8s_be;elsesbc_enc_process_input =priv->enc_state.sbc_enc_process_input_8s_le;} else {if (sbc->endian == SBC_BE)sbc_enc_process_input =priv->enc_state.sbc_enc_process_input_4s_be;elsesbc_enc_process_input =priv->enc_state.sbc_enc_process_input_4s_le;}priv->enc_state.position = sbc_enc_process_input(priv->enc_state.position, (const uint8_t *) input,priv->enc_state.X, priv->frame.subbands * priv->frame.blocks,priv->frame.channels);samples = sbc_analyze_audio(&priv->enc_state, &priv->frame);if (priv->frame.mode == JOINT_STEREO) {int j = priv->enc_state.sbc_calc_scalefactors_j(priv->frame.sb_sample_f, priv->frame.scale_factor,priv->frame.blocks, priv->frame.subbands);framelen = priv->pack_frame(output,&priv->frame, output_len, j);} else {priv->enc_state.sbc_calc_scalefactors(priv->frame.sb_sample_f, priv->frame.scale_factor,priv->frame.blocks, priv->frame.channels,priv->frame.subbands);framelen = priv->pack_frame(output,&priv->frame, output_len, 0);}if (written)*written = framelen;return samples * priv->frame.channels * 2;
}SBC_EXPORT void sbc_finish(sbc_t *sbc)
{if (!sbc)return;free(sbc->priv_alloc_base);memset(sbc, 0, sizeof(sbc_t));
}

关闭文件。

done:if (fd > fileno(stderr))close(fd);
}

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【蓝牙sbc协议】sbc源码阅读笔记（一）——编码过程

sbcenc.c

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